α-Synuclein is a neuronal protein, which originally has been associated with neuronal plasticity during Zebra finch song learning. Although its role at the molecular level is at present largely elusive it appears to have lipid bi-layer (or membrane) with binding properties important for preserving proper transport of neurotransmitter vesicles to the axonal ends of neurons presumably to ensure proper signalling at the synapse. Apart from its physiological role in brain cells, human α-synuclein also possesses pathological features that underlies a plethora of neurodegenerative diseases including Parkinson's disease, diffuse Lewy body disease, traumatic brain injury, amyotrophic lateral sclerosis, Niemann-Pick disease, Hallervorden-Spatz syndrome, Down syndrome, neuroaxonal dystrophy, multiple system atrophy and Alzheimer's disease. These neurological disorders are characterised by the presence of insoluble α-synuclein polymers or aggregates usually residing within neuronal cells, although in the case of Alzheimer's disease α-synuclein (or proteolytic fragments thereof) constitutes the non-amyloid component of extracellular “amyloid-β plaques”. It is widely believed that the amyloidogenic properties α-synuclein disrupt cellular integrity leading to dysfunctioning or death of affected neurons resulting in cognitive and/or motoric decline as it is found in patients suffering from such diseases. The aggregation of α-synuclein is at present very poorly defined, but constitutes most likely a multi-step process wherein self-polymerization of α-synuclein into insoluble aggregates is preceded by the formation of soluble protofibrils of α-synuclein monomers. Self-association may be triggered by the formation of alternative conformations of α-synuclein monomers with high propensity to polymerize. Several studies using neuronal cell lines or whole animals have shown that formation of reactive oxygen species (hereinafter abbreviated as ROS) appear to stimulate noxious α-synuclein amyloidogenesis. For instance paraquat (an agent stimulating ROS formation within the cell) has been recognized as a stimulator of α-synuclein aggregation. Like in animals, exposure to paraquat is believed to induce the formation of synuclein inclusions, and consequently neurodegeneration, especially of dopaminergic neurons in humans. Dopaminergic neurons appear to be particularly sensitive because the concurrent dopamine metabolism may on the one hand contribute significantly to the oxidative stress load but may on the other hand result in kinetic stabilisation of highly toxic protofibrillar α-synuclein species by dopamine (or its metabolic derivatives). Parkinson's disease is characterised by a selective loss of dopaminergic substantia nigra cells and therefore treatment of animals (or neuronal cells) with paraquat is a common well-accepted experimental set-up for studying synucleopathies, in particular Parkinson's disease.
Apart from ROS, mutations in the coding region of the α-synuclein gene have also been identified as stimulators of self-polymerization resulting in early disease onset as it is observed in families afflicted by such mutations. Finally, increased expression of α-synuclein also promotes early disease onset as evidenced by a duplication or triplication of the α-synuclein gene in the genome of some individuals. The molecular mechanism by which α-synuclein self-association triggers cellular degeneration is at present largely unknown. Although it has been speculated that insoluble aggregates affect cellular integrity, it has recently been suggested that soluble protofibrillar intermediates of the aggregation process are particularly toxic for the cell as opposed to mature insoluble fibrils which may be inert end-products or may even serve as cytoprotective reservoirs of otherwise harmful soluble species. Therapeutic attempts to inhibit formation of insoluble aggregates may therefore be conceptually wrong, possibly even promoting disease progress.
While the identification of pathological α-synuclein mutations unequivocally revealed a causative factor of a plethora of neurodegenerative disorders, treatments ensuring suppression of toxic α-synuclein amyloidogenesis are presently not available. Only symptomatic treatments of Parkinson's disease exist, which aim e.g. at increasing dopamine levels in order to replenish its lowered level due to degeneration of dopaminergic neurons, for instance by administrating L-DOPA or inhibitors of dopamine breakdown. Although such treatments suppress disease symptoms to some extent, they are only temporarily effective and certainly do not slow down ongoing neuronal degeneration.
Thus there is a need in the art for designing new drugs for therapeutic treatments that target the underlying molecular mechanism of α-synuclein related pathologies in order to reduce neuronal cell death and/or degeneration.